An internal combustion engine is provided with a canister circuit, which has the function of recovering the fuel vapours which are produced in the fuel tank and of introducing such fuel vapours into the cylinders in order to be burnt; this prevents the fuel vapours which are produced in the fuel tank from leaking from the fuel tank (specifically when the fuel filler cap is opened for refueling) and being freely dispersed into the atmosphere.
In an aspirated internal combustion engine (i.e. without supercharging), the canister circuit comprises a recovery pipe which originates in the fuel tank and ends in the intake manifold plenum and is adjusted by a canister solenoid valve of the on/off type. Atmospheric pressure is essentially present inside the fuel tank, while a slight vacuum is present in the intake manifold plenum determined by the intake action generated by the cylinders; consequently, when the canister solenoid valve is open, the gasoline vapours are naturally sucked down along the recovery pipe from the fuel tank into the intake manifold plenum.
A supercharged internal combustion engine is provided with a turbocharger (either a turbocharger actuated by the exhaust gases or a volumetric turbocharger actuated by the drive shaft) which in some moments compresses the aspirated air in order to increase the volumetric efficiency. By effect of the action of the turbocharger in a supercharged internal combustion engine, in the intake manifold plenum there may be either a slight vacuum determined by the intake action generated by the cylinders (turbocharger not running) or an overpressure determined by the compression action of the turbocharger (turbocharger running). Consequently, in a supercharged internal combustion engine, the canister circuit is more complex because, downstream of the canister solenoid valve, the recovery pipe has a fork adjusted by a one-way membrane valve; one branch of the recovery pipe fork leads to the intake manifold plenum, while the other branch of the recovery pipe fork leads to an intake manifold upstream of the turbocharger. When the turbocharger is not running, there is a slight vacuum determined by the aspiration action of the cylinders in the intake manifold plenum, while there is atmospheric pressure in the intake pipe upstream of the compressor; in this situation, the one-way membrane valve allows the gasoline vapours to enter the intake manifold plenum directly. When the compressor is running, there is an overpressure determined by the compression action of the compressor in the intake manifold plenum, while there is a vacuum determined by the intake action of the compressor in the intake pipe upstream of the compressor; in this situation, the one-way membrane valve allows the gasoline vapours to enter the intake pipe upstream of the compressor.
By effect of the presence of the fork in the recovery pipe and of the one-way membrane valve, the canister circuit of a supercharged internal combustion engine has various external components (tubings and pipe fittings) and is relatively complex and extended; consequently, the assembly of the canister circuit of a supercharged internal combustion engine takes a relatively long assembly time and thus determines a non-negligible assembly cost.